The present invention relates generally to power supplies for electrical loads, and more particularly to a transformerless power supply of the electronic switching type that can be driven directly by alternating current from a commercial power line to energize a low voltage requirement direct current load. The particular type of supply described herein does not provide isolation between the AC mains and the DC load. The invention is intentionally comprised of a minimum number of low cost components to cost effectively satisfy the power supply functions required of the electronics typically used in appliance control and electronic utility metering applications.
Most semiconductor-based electronic circuits require a low voltage regulated DC source as a power supply. To operate such electronic circuits from the commercial AC mains, some type of power supply circuit must be used to convert the high AC mains voltage input to a regulated DC low voltage output. To date, such line driven power supplies have been primarily comprised of five well known types.
In the first type, the AC mains voltage is applied directly to a transformer or autotransformer, which is designed to operate at the frequency of the AC mains, typically 60 Hz. The transformer efficiently steps down the AC mains voltage to a lower voltage level. The resulting low voltage AC is then rectified, filtered and sometimes further regulated to produce the desired low voltage DC output. The transformer or autotransformer in this type of supply usually determines the cost, volume and weight of the power supply.
In the second type of prior art supply, commonly referred to as an off-line switchmode power supply, the AC mains is directly rectified and filtered. The resulting high DC voltage is then applied to a high frequency switching circuit. Many such high frequency switching topologies have been described, such as the buck, flyback, and forward converters. All of these topologies require a magnetic element, either an inductor or a transformer, to transform the high voltage DC to a lower voltage. Because of the high switching frequency, usually 10 KHz to 500 KHz, the magnetic element is usually physically smaller than those used in the aforementioned 60 Hz design, but exotic magnetic core materials and complex construction often result in a costly component. Off-line switchmode converters can offer high power efficiency, but the high input voltage to the converter further complicates the design of the magnetics. The necessity of high performance switching circuitry may also prohibitively increase the cost of such a converter.
The third type of line driven supply is a reactive divider, which functions in a well known manner to divide the AC mains voltage into larger and smaller portions using inductors or capacitors. The smaller portion of the AC mains voltage is then rectified and filtered to supply the low voltage power to the load. Reactive dividers may operate at higher efficiencies, but the reactive elements required to handle the high AC voltages and currents are both costly and bulky.
The fourth type of line driven supply is a resistive divider. The AC mains voltage is divided into larger and smaller portions using resistive elements, and the smaller portion is rectified and filtered to produce a low voltage DC output. It is also a common technique to first rectify the AC mains to produce a high DC voltage, which is then applied to a resistive divider. The well known series-pass regulator circuit is an example of such an approach. A series-pass regulator uses a controllable device, such as a transistor, as a controllable series resistance, or "pass element", and the load resistance functions as the second half of the resistive divider. Series-pass regulators are most efficient when the voltage drop across the series element is a small fraction of the output voltage, which occurs when the input voltage is close to the output voltage. In the intended application, the average AC mains voltage is much larger than the desired DC output voltage, so most of the high voltage DC would have to be dropped across the pass element, resulting in low power efficiency, hence considerable power dissipation. A shunt regulator, such as a resistor/Zener combination, would waste even more power because both shunt and load current must flow through the series element.
The fifth type of line driven supplies draw current from the AC mains input only during selected time segments of the AC input cycle. This type of supply relies upon the fact that the AC mains voltage waveform is a continuous function, and a serial switching element in such a supply may be connected to the output synchronously with the AC input at the proper point in the AC input cycle to produce the desired output. Power supplies containing Triacs or SCR's often use this phase angle modulated switching approach to produce average outputs much lower than that of the AC mains input, sometimes in conjunction with magnetic elements. The invention described herein operates on this principle of synchronous conduction without the use of magnetic elements.
The U.S. Patent references cited in this document all disclose inventions operating upon the principle of interrupting current flow between the AC mains input and an output charge storage capacitor using a serial semiconductor switch whose switching threshold points are determined whole or in part by the instantaneous voltage appearing at the AC mains input to the power supply.
Lewis, U.S. Pat. No. 4,001,668, issued Jan. 4, 1977, discloses a power supply designed to provide a low voltage DC power source for an electric shaver motor using the AC mains directly as the input. The circuit described therein half wave rectifies the AC mains, and opens a serially connected semiconductor switch whenever the AC mains voltage exceeds a predetermined threshold. In this manner, a high power efficiency is maintained throughout a specified 100 VAC to 270 VAC range of AC input voltages. The circuit provides no input transient protection. The circuit monitors only at the instantaneous AC input voltage, and does not use feedback to achieve precise regulation of the output voltage. In the event of an output overload or short-circuit, no positive method is provided to prevent excessive and potentially destructive power dissipation from occurring in components of the circuit.
Sanders, U.S. Pat. No. 4,685,046, issued Aug. 4, 1987 discloses a power supply incorporating a Zener diode clamp in the output stage in an open-loop fashion to achieve improved precision in the regulation of the output voltage. In the preferred embodiment, the switch functions as the pass element, and therefore could dissipate considerable power. Because the semiconductor switch is connected directly to one side of the AC line, effective protection of the switch from high-voltage line transients would be difficult to implement. Sanders specifies his circuit only for 120VAC operation. No input transient protection nor output overload and short-circuit protection is provided.
Roy, U.S. Pat. No. 4,641,233, issued Feb. 3, 1987 discloses a power supply incorporating output voltage feedback from the output to maintain good output voltage regulation. Again, no input transient protection nor output overload and short-circuit protection is provided.
Fukushima, U.S. Pat. No. 5,307,257, issued Apr. 26, 1994 discloses several embodiments of a power supply in which the switch is controlled by the average of the AC mains voltage, the instantaneous value of the AC mains voltage, and the output voltage. Fukushima offers nine embodiments of the invention with varying degrees of complexity, but makes no claims regarding the importance of selecting switching thresholds to achieve desirable attributes of the invention, such as output current limiting. No input transient protection nor output overload and short-circuit protection is provided.
An object of the invention is to provide an efficient, low cost power supply which accepts directly the commercial AC mains voltage as input and provides a regulated low voltage DC output.
Another object of the invention is to provide a power supply operated from the AC mains without transformers or other external voltage reducing means.
Still another object of the invention is to provide a predetermined stable DC output voltage which is insensitive to the magnitude of the incoming AC mains voltage over a wide range of input AC mains voltage.
A further object of the invention is to provide stable output overload and short circuit protection for the benefit of both the power supply and any circuit load which may be powered by the power supply. The performance of said protection shall be substantially invariant with changes in AC input voltage, ambient operating temperature and DC load current.
Another object of the invention is to provide an integral and cost-effective method of absorbing deleterious voltage transients routinely appearing on the commercial AC mains. Such transients are typically caused by lightning strikes or the connection and disconnection of other circuits occurring on the commercial AC mains.
Other and further objects of the invention will become obvious upon reading the below described specification.